Yes, you can charge your iPad Pro with the 87W Power Adapter. Your iPad will only "ask" the adapter for so much current, and the adapter will not give more than that. Depending on if your iPad is equipped with fast charge, it may charge your iPad faster.

Apple's USB-C Power Supplies utilize PD. This is a USB-C power delivery protocol that allows for multiple power output profiles. If you look at the side of your power supply you can see all of the profiles listed. There is a profile for 5.2V 3A which will deliver essentially 15W at a safe voltage to a mobile device. (MFi standards for an iPhone are not to exceed 5.25V).

New iPhone and iPad models can accept higher than the prior MFi Maximum 12W power (5.2V 2.4A). In these scenarios, the iPhone can "quick charge", or can re-charge about 50% of the battery in about 30 mins.

No worries about damaging the iPhone because the power supply requires essentially a "handshake" with the mobile device (iPad, Macbook, MacBook Pro) to pass higher voltage. If the device cannot handle the higher current / voltage PD profiles, the power supply will revert to the lowest profile.

Hopefully, this helps clear this up. USB-C is a very cool standard and I think because it is new there is not much press around its versatility. Could be an opportunity for Apple to market up these features and educate users while also building more value around its products ;)

This might be a dumb question, but I'm wondering if I can use the three prong extension cord off my old (2015) macbook's power cable. I'm assuming there is no special amp/wattage etc. considerations to be made for this addition?

Ideally, you should use the 87W charger with a proper 5A/100W cable, but lower-powered chargers will also work.

Keep in mind that running a CPU or GPU intensive task (like encoding a video) with an external monitor attached can easily draw 60W, so anything less than the 87W adapter will result in curtailed charging speed wile doing computationally intensive tasks.

On the other hand, using lower-powered chargers can provide portability and flexibility advantages.

Assuming that the 2017 is the same as the 2016, you can actually (slowly) charge your MacBook Pro 15" with any decent apple-coded 5V, 2.4A 12W charger (like an iPad charger) via a USB-A to USB-C cable. Such a charger could also provide most of the power needed for lightweight tasks, like checking email, without drawing much power from the internal battery.

The MBP 15" also works with a 15W (5V @ 3A) USB Type-C chargers, and many higher-powered USB Type-C chargers with USB-PD support. I'm running mine off a 30W USB-PD capable external battery right now. I've also powered it with a 27W capable USB-PD compliant car charger and a (somewhat) pocketable 27W USB-PD wall-wart.

i.e. If left plugged in to a power outlet when not connected to a device will it still draw current?

I'll be using it to charge a Touchbar MBP 15". With my previous MBP I would charge it from my Thunderbolt monitor, which only supplied current when the MBP was connected. Since a MagSafe to USB-C adapter is not available I will have to use this charger instead but will need to leave it connected to the wall socket behind my desk and I don't want it to be wasting current through the transformer all day while I'm out.

As others have said, yes, you can use it with MacBook's that ship with lower powered USB-C adapters.

However, it's not true, as one of the answers states, that you shouldn't do the reverse, that you shouldn't use a lower powered adapter with a MacBook that ships with a higher-powered adapter.

The adapters that ship with MacBooks are sized to provide enough power to both charge the battery quickly, and run the laptops at or close to their power limit when doing intensive computational-tasks (like encoding video). Using the specified wattage will ensure that you can get the most out of your MacBook in all conditions.

The flip side is that much of what most people do on their laptops doesn't require the full power. My 15" MBP will use ~60W when encoding a video, and if the battery is also low, it will draw the full 87W available from the stock adapter, but right now, as I type this and browse the web, it is averaging about 10W. Under such conditions a 61W, or even a 45 or 30W adapter would be enough to power my use of the computer and recharge the battery. The battery wouldn't recharge at full speed with the 45 or 30W adapter, but it would still charge at a decent rate.

So, yes, ideally, you should use your MacBook with a charger that matches or exceeds the power of the adapter it was provided with, but using a good, but lower-powered adapter won't hurt your computer, and can come in handy if you want something lighter-weight for travel, or you left your adapter behind and can only borrow a lower-powered adapter.

I tried to charge my iPhone 8 from 1% battery to 100% with 87W power adapter and USB-C to Lighting 1 meter long Apple cable. It took 1 hour and 35 minutes. Not bad when I am in rush and need to charge ASAP. However I am using good old 1A chager from the iPhone box on regular basis in order to save the battery life.

With the MacBook Pro it can depend on what model you have for charging. This one you selected is for the 15inch MacBook Pro 2016 or later models They would have the Thunderbolt 3 ports. It would be awesome to have a 12Vt charger however it wouldn't have enough power to push it.

If you are referring to the new 2016 MacBook Pro, then yes, it can be used period, not just 'in a pinch'. It will work perfectly fine--it is just that this is a more expensive adapter since it is higher power, a 13" 2016 MBP will only charge at 61W, not 87W, but it is safe and works fine.